late 12c., "gnashing the teeth," from grind (v.). The sense "steady, hard work" first recorded 1851 in college student slang (but cf. gerund-grinder, 1710); the meaning "hard-working student" is American English slang from 1864.

The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D.Copyright (C) 2007 by HarperCollins Publishers.Cite This Source

re-ground in the Bible

(Ex. 32:20; Deut. 9:21; Judg. 16:21), to crush small (Heb. tahan); to oppress the poor (Isa. 3:5). The hand-mill was early used by the Hebrews (Num. 11:8). It consisted of two stones, the upper (Deut. 24:6; 2 Sam. 11:21) being movable and slightly concave, the lower being stationary. The grinders mentioned Eccl. 12:3 are the teeth. (See MILL.)